There is a tremendous and growing interest in the research of organic-based optoelectronic materials for organic light-emitting diodes (OLEDs) which are promising elements for the next generation of flat-panel displays. Owing to its thin-film, light-weight, fast-response, wide-viewing-angle, high-contrast, and low-power attributes, OLED promises to be one of the major flat-panel-display technologies that can compete with the now-dominant liquid-crystal displays (LCDs) in the new millennium. Phosphorescent metal complexes that can be incorporated into OLEDs are under active investigation in academic and industrial R&D laboratories. This interest arises from the potential to increase device efficiency, relative to devices containing all-organic emissive materials, which is usually limited to an external quantum efficiency of around 5% based on singlet-state fluorescent materials. Phosphorescent emitters with heavy metal ions allow for circumvention of this limitation if the excitons generated by hole-electron recombination reside at a site where efficient spin-orbit coupling leads to strong singlet-triplet state mixing. Since both electrogenerated singlet and triplet excitons can be harvested for light emission in these complexes, the internal quantum efficiency of phosphorescent emitters can approach 100% theoretically (Adachi et al., J. Appl. Phys. 2001, 90, 5048).
While this research field has many interesting and novel opportunities on account of its huge market share in next generation flat-panel display technologies, it is now identified that molecules or polymers with specific functions such as hole transportation, electron transportation, emission and thermal stability are ideal for this purpose. Current focus and challenge for OLEDs lie in the optimization of EL cell structures and the use of electrophosphorescence for the improvement of device performance. The latter work has relied on the use of spin triplet states toward light emission (i.e. triplet-harvesting). Hole-electron recombination in OLEDs produces both singlet and triplet excitons within the molecular thin film. For most fluorescent compounds, only the singlet state is emissive, leading to a significant limitation in the OLED efficiency. An excellent method to efficiently harvest energy from the triplet states involves the incorporation of third row heavy metals and these metal complexes permit the opening of an additional radiative recombination channel because of the associated strong spin-orbit coupling, resulting in a harvesting of up to nearly 100% of the excited states to photon creation.
This line of research would set a new benchmark for the theoretical efficiency limit, namely 75% for triplet emission as opposed to the 25% for singlet emission. The scope and diversity of studies on metal-organic phosphors in the realm of materials science have continued to expand and the interest in these materials and their electrophosphorescent properties spans the entire globe. Over the years, significant advances were made in this area and there is a great potential to excel in the exploration of triplet emitters for organic molecular optoelectronics that can be developed for use in display technology.
Heavy metal compounds of iridium(III) and platinum(II) can be used as emissive traps or dopants in OLEDs, leading to unprecedented quantum efficiencies for these devices. Both the EL efficiency and the emission wavelength of these electrophosphor-doped devices are strongly influenced by the structural features of organic ligand chromophores which are generally 2-arylpyridine derivatives or other nitrogen-containing heterocycles (abbreviated as C^N). Although red-, green-, and blue-emitters with excellent color purity and sufficient luminous efficiency are required for full-color display applications, there is also a continuing demand for monochromatic emitters that afford a bright color, such as yellow, orange or light blue, for multiple-color display purposes.
On the other hand, white organic light-emitting diodes (WOLEDs) have drawn much recent attention in the scientific community because of their potential use in display backlights, full color applications, as well as in solid-state lighting purposes. WOLEDs make attractive candidates as future illumination sources over the conventional incandescent bulbs and fluorescent lamps for several reasons, including compact size, the suitability for fabrication on flexible substrates, low operating voltages, and good power efficiencies. In particular, WOLEDs employing phosphorescent materials have led to significant improvements in efficiency, targeting backlights for full color active-matrix displays combined with color filters. While electrophosphorescent OLEDs have been shown to have very high external quantum efficiency when used for monochromatic light emission, their incorporation into a white emitting device should similarly lead to high-efficiency WOLEDs.
The present work called for a highly topical area in the development of a novel series of heavy metal organometallic triplet emitters containing diarylaminofluorene components that can be used as efficient phosphorescent dopants in high-efficiency OLEDs applications. Fluorene-based chromophores hold great promise as highly stable and efficient emissive cores in the synthesis of useful metal complexes (Wong, Coord. Chem. Rev. 2005, 249, 971). Fluorene derivatives are attractive candidates for optoelectronic applications because of their good thermal and chemical stability, high emission quantum yields and the ease of functionalization of the fluorene at 2-, 7- and 9-positions. However, most of the related reports involving fluorene cores have been concentrated on polymer-based devices where the fluorene-containing phosphor is processed with a polymer host by the spin-coating method (Wu et al., J. Mater. Chem. 2005, 15, 1035 and Gong et al., Adv. Mater. 2003, 15, 45). It is also known that a large hole-injection barrier for organic fluorene-based molecules often limits their device efficiency. Since triphenylamine groups are known to possess a superior hole-transport mobility, and glass-forming aromatic amino derivatives, such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB), are widely used for the preparation of hole-transporting materials used in OLEDs, we envision that triarylamines or their derivatives can be incorporated into the fluorene nucleus to improve the hole-injection/hole-transporting properties and morphological stability. The molecular design of amorphous organometallic phosphors capable of efficient visible emission in OLEDs is highly challenging and it is desirable to design new ligand systems for heavy metal complexes with good amorphous properties and improved functional properties. To our knowledge, there are no literature reports exploring arylamine-substituted fluorene-based electrophosphors for small-molecule OLEDs. The major impetus for the present invention is stimulated from the contention that the use of arylamino-fluorene ligand chromophores can improve the morphological stability and charge-transport properties of metalated phosphors.
Others have attempted to provide phosphorescent compounds and devices made from those compounds. U.S. Pat. No. 6,835,469 B2 (Kwong) discusses phosphorescent organometallic complexes comprising phenylquinolinato ligands, and high efficiency organic light emitting devices comprising these compounds.
U.S. Pat. No. 6,902,830 B2 (Thompson) provides organic light emitting devices wherein the emissive layer comprises a host material containing an emissive molecule. The emissive molecule is a phosphorescent organometallic complexes, including cyclometalated platinum, iridium and osmium complexes.
U.S. Pat. No. 6,573,651 B2 (Adachi) is directed to OLED structures comprising an anode layer, a hole injecting layer (HIL) in direct contact with the anode layer, an emissive organic electron transporting layer (ETL) in direct contact with the hole injecting layer, and a cathode layer in direct contact with the emissive organic electron transporting layer. The emissive organic electron transporting layer comprises an organic electron transporting material and an organic hole-trapping emissive material, for example, an organic phosphorescent material that produces emission from a triplet excited state of an organic molecule.
U.S. Pat. No. 6,670,645 B2 (Grushin) relates substituted 2-phenylpyridines, phenylpyrimidines, and phenylquinolines electroluminescent Ir(III) compounds and devices that are made with the Ir(III) compounds.